1. Field of the Invention
This invention relates to a method for computing the graphic area ratio of picture images on printing plates effectively and efficiently, and an apparatus for performing such a task.
2. Background Art
Printing plates are classified into three broad types depending on the techniques used in preparing the printing plates: relief surface type, intaglio or depressed surface type and planographic type. The present invention is concerned with the planographic type. This type of printing plates is best suited for color printing or large area printing work, and the printing plates are made of aluminum. These plates are coated with a photo-sensitive emulsion sensitive to a particular wave length (UV, ultra violet), and such plates are called PS (presensitized) plates.
In coloring printing, the usual practice is to prepare four plates P consisting of colors Y (yellow), M (magenta), C (cyanogen) and BK (black) for one sided printing and eight plates for two sided printing. There are cases in which several pages are arranged on one plate, and depending on the number of printed pages involved, many printing plates are required.
On the printing plates P are photographically exposed picture patterns by means of netting positive films, which contain all the necessary information to be printed, such as the picture images, lines and letters. All emulsions except such printing information are removed, and the remaining information emulsions are protected by hardening of the emulsion by such means as burning (developing) and by coating.
In the meantime, the printing ink for the printing plates is supplied through ink keys which are placed at a fixed distance on the printing machine. The plates are divided into sections of a given width (for example, 30 mm) along the printing direction, each section corresponding with an ink key. The amount of ink to be supplied to each section is controlled by the picture patterns to be printed. The area occupied by various patterns is controlled by the relative areas of the pattern to be printed. The larger the area occupied by a certain pattern, the greater the amount of ink which must be supplied to print that pattern. On the other hand, when the area of the pattern is small, the amount of ink required to print that pattern must also be kept correspondingly small.
Accordingly in such a printing process, it is necessary to control the amount of printing ink depending on the relative pattern ratio of a particular graphic element within the overall graphic pattern. If there is a supply mismatch, there will be fading in the region of insufficient supply and smearing in the region of oversupply.
It has been the practice to measure the relative area in each of the ink keys, after the completion of the development stage. The value of this relative area is obtained according to the difference in the relative reflectivities between the regions with and without the emulsion. The measurement method utilizes the characteristics of the developed plates to reflect more light from the regions stripped of emulsion compared with the emulsion coated region. Reflectance measurements are made with a laser beam, which scans the surface of the plate P, and the intensity of the reflected beam is measured by a light meter. The varying intensities of the reflected beams from the surface indicates whether the reflection is from a patterned region or not, and enable determination of the area ratios of various regions to correspond with the respective ink keys.
We now explain the process of affixing the printing plates to printing roller bodies and the printing process with reference to FIG. 16. The measured plate P is wound and affixed on a roller body 2. The surface is then flooded with water supplied from a water transfer roller (not shown). Of the two types of materials on the plate surface, the aluminum surface is hydrophilic while the emulsion is lipophilic. Therefore, water wets the exposed aluminum regions and not the patterned regions coated with the emulsion. Next, printing ink is supplied to the plate surface through the inking roller 3, which transfers ink onto those regions of the plate which are coated with the emulsion but not to those regions of exposed aluminum wet with water. The ink patterns are transferred onto an elastic rubber blanket 4, from which the ink is transferred onto the surface of a printing object 5, such as paper, and the printing process is completed. This process is repeated for all the printing plates.
The area ratio computing apparatus is used with a plate held in the roller body, and therefore the apparatus has to be a correspondingly large apparatus, requiring a large working space. It is also equipped with a laser scanning mechanism and other mechanical moving parts, and is susceptible to breakdown problems associated with the hardwares. The use of emulsions to treat the surface often leads to problems of non-uniform surface quality, which leads to further problems of scatter in the reflectance method of measuring the surface area. The differences in the surface quality (coloring, flatness and different surface treatments), caused by differing lots and processing, necessitates the resetting of parameters for every measurement. When the surface quality is variable within a plate, the reflectance for a pattern is not uniform, and causes inaccuracies in the area integration results. The ideograms and lines and other fine details require high resolution and are difficult to measure by the conventional techniques. Furthermore, printing usually requires a large number of plates P, and the unavoidable problems of manual setting of the plates and area measurements not only require a large expenditure of time and manpower, but also cause the associated problems of accidents and breakage due to manual handling. The overall result is a serious loss in printing efficiency.